Inkjet inks used for inkjet printing system are generally classified into aqueous inks and oil inks.
Aqueous inks have small dot gain compared with oil inks and thus are suitable for obtaining images high in resolution and density, and generally used in inkjet printers for office and domestic uses. Also, in order to prevent bleeding of an aqueous ink so as to achieve image printing with higher resolution, there has been proposed a printing method in which a treatment solution reactive with the aqueous ink is first ejected onto a printing medium, and then the ink is ejected so as to overlay the treatment solution, whereby coloring materials contained in the ink aggregate (refers to Patent Document 1).
However, aqueous inks are small in dot gain, and thus are unsuitable for low resolution printing. The printing method using an aqueous ink together with the above treatment solution is further unsuitable for low resolution printing because it makes dot gain further smaller. Also, when aqueous inks are printed on plain paper, they easily provide prints with curls, and thus are inferior in conveyance of prints and unsuitable for high-speed printing.
In contrast, oil inks have large dot gain compared with aqueous inks, and do not provide prints with curls even when they are printed on plain paper, and thus are suitable for high-speed printing with low resolution. Also, oil inks volatize solvents less than aqueous inks, and thus are advantageous in that clogging in ink nozzles is unlikely to occur, thereby reducing frequency of cleaning of ink nozzles, and thus are suitable for high-speed printing, particularly high-speed line head inkjet printing system (Patent Document 2).
However, oil inks are defective in that they are low in printing density and easily cause strike through or bleeding since the coloring material and the solvent are highly compatible with each other so that the coloring material easily permeates together with the solvent into printing media but can hardly stay on a surface of the printing media.
It has been already proposed that an emulsion ink is used in an inkjet printer in order to solve the problems of printed dot size and printing density that both aqueous and oil inks possess (Patent Document 3).
However, the conventional emulsion ink has only achieved a property which is intermediate between aqueous inks and oil inks, but does not have much difference from oil inks in printing density, in particular.    [Patent Document 1] JP-A-H05-202328    [Patent Document 2] JP-A-2005-350563    [Patent Document 3] JP-A-2006-56931